| --- |
| page_title: cidrnetmask - Functions - Configuration Language |
| description: |- |
| The cidrnetmask function converts an IPv4 address prefix given in CIDR |
| notation into a subnet mask address. |
| --- |
| |
| # `cidrnetmask` Function |
| |
| `cidrnetmask` converts an IPv4 address prefix given in CIDR notation into |
| a subnet mask address. |
| |
| ```hcl |
| cidrnetmask(prefix) |
| ``` |
| |
| `prefix` must be given in IPv4 CIDR notation, as defined in |
| [RFC 4632 section 3.1](https://tools.ietf.org/html/rfc4632#section-3.1). |
| |
| The result is a subnet address formatted in the conventional dotted-decimal |
| IPv4 address syntax, as expected by some software. |
| |
| CIDR notation is the only valid notation for IPv6 addresses, so `cidrnetmask` |
| produces an error if given an IPv6 address. |
| |
| -> **Note:** As a historical accident, this function interprets IPv4 address |
| octets that have leading zeros as decimal numbers, which is contrary to some |
| other systems which interpret them as octal. We have preserved this behavior |
| for backward compatibility, but recommend against relying on this behavior. |
| |
| ## Examples |
| |
| ``` |
| > cidrnetmask("172.16.0.0/12") |
| 255.240.0.0 |
| ``` |
